Sadly this experiment would cost in the high tens of thousands of dollars. We may try with titanium some day. That would only be ten thousand dollars.
Where'd you get the magnesium cube?
Osmium is about twice as dense so yeah that would still be shippable at around 60 lb... I don't think that's even tens of thousands at that point? Isn't it going to be in the millions? I'm just thinking that like if you went for gold instead, 50 lb is 800 oz, at $3300/oz these days doing this with gold is $2.64M, no? And surely osmium blows gold out of the water by like 10x, right?
Jokes on you when the $100 of insurance is all you get back from the post office...
Edit: a sibling comment points out that you can probably do this under $100 with tungsten too and get up into that 50lb+ range.
DU is harmless unless you eat or breathe it but alas it's now illegal to possess more than a minute quantity of it.
Tungsten is actually slightly denser and it has the advantage of being obtainable.
That's why our use of big lumps of DU as door stops was considered "safe" (at that time several years ago) but in labs where people machined the stuff they were a lot more careful.